The subject invention relates generally to the measurement of surface properties of electron-emissive materials and, more particularly, to a method for predicting the suitability of various materials for use in electron-emissive cathodes.
Modern dispenser cathodes which are used in most microwave tube applications are usually composed of a metal matrix which has been impregnated with an alkaline earth metal compound. This compound, when heated, dispenses material onto the surface of the matrix metal which then forms a layer on the surface which makes the surface a good electron emitter. This is accomplished due to a lowering of the electronic surface barrier, that is, the surface work function.
The material which forms the layer on the surface is an oxide of the alkaline earth metal (e.g., Barium oxide). A crucial factor in the surface work function lowering is the dipole moment of the molecules of the surface layer material. This dipole is known to be different for different combinations of materials. It would therefore be desirable to have a means for predicting the dipole of various combinations of cathode materials without having to fabricate actual cathodes from those materials and then measure the thermionic emission properties of each one.
The charge transfer that takes place between the alkaline earth metal and the oxygen atoms gives an indication of the magnitude of the dipole moment when the surface layer material is adsorbed on a particular cathode substrate. The inventors have discovered that the extent of this charge transfer can be determined by analysis of the Auger electron energy spectrum of the cathode material.
Electron spectroscopy, and in particular Auger electron spectroscopy, are well-known methods for analyzing surface properties of materials. For example, in "Analysis of Materials by Electron-Excited Auger Electrons", L. A. Harris, J. of Appl. Phys., Vol. 39, No. 3, 1419 (Feb. 15, 1968), it is disclosed that the compositions of surface materials may be analyzed by observing the energy spectrum of Auger electrons emitted by a sample which has been bombarded with an electron beam. The energy spectrum observed is characteristic of the composition and may be used to identify its component elements.
U.S. Pat. No. 3,361,238, MacDonald, discloses a method based on Auger electron spectroscopy, for measuring the electric potential on the surface of a material. MacDonald obtains direct quantitative measurement of the electronic potential by measuring the shift in energy levels of the Auger peaks between two consecutively analyzed points on the material surface.
U.S. Pat. No. 3,965,351, Strongin et al., discloses a method of Auger spectroscopy of dilute alloy surfaces. According to Strongin et al., the electron beam scan is alternated between a pure reference sample and a sample of the material under test. The electrical signal produced at the reference sample is processed such that it is subtracted from the signal produced at the test material sample. The difference signal then represents an Auger spectrum of the trace impurity.
Although all of the above references utilize Auger spectrum analysis, none of them either concerns or discloses methods for measuring surface dipole strength or work function lowering. These references are primarily directed to identification of materials. However, there have been some efforts to measure surface work function parameters.
U.S. Pat. No. 3,337,729, Thomas et al., discloses a method and apparatus for investigating the variation in surface work function of a material. The method is directed to scanning the surface of a sample of the material with an electron beam. The electrical signal produced by the scanning electron beam at the target material is then processed and visually displayed. The shadings in the visual display are representative of the variations in the surface work function across the material surface.
U.S. Pat. No. 4,142,145, Haas et al., discloses a method for measuring surface work function which utilizes low-energy electron reflections to determine the electron affinity and to locate the conduction-band edge relative to the Fermi level at the surface of single-crystal semiconductor material. A beam of electrons is directed onto the material surface. The current collected by the semiconductor is then analyzed as a function of beam energy in order to determine both the position of the conduction band edge with respect to the Fermi level and the electron affinity. The surface work function can then be determined from these two quantities.
Although both of the above cited references disclose methods for measuring surface work function parameters, neither discloses a method for predicting surface work function lowering by measuring charge transfer from interatomic Auger transitions.
In light of the foregoing discussion it appears that there is presently no known method for estimating surface work function lowering due to the formation of a dipole moment, much less one utilizing Auger spectrum analysis.